Such a heating device is known, for example, from German Published Patent Application No. 198 34 276 for heating a sensor element which is used in a gas sensor for exhaust gas analysis in internal combustion engines. The sensor element has solid electrolyte layers and electrodes, as well as a heating device having a stratified design. The heating device is situated on a solid electrolyte layer or between one solid electrolyte layer and another solid electrolyte layer. The heating device contains a heating element made up, for example, of an electrical resistor layer and an insulator in which the heating element is embedded. The insulator is essentially made of aluminum oxide. The insulator electrically insulates the heating element against the solid electrolyte layers and the electrodes, as well as against electron and ion conduction, so that the function of the sensor element is not impaired by the operation of the heating device.
The heating device is manufactured by applying a lower insulator layer, the heating element, and an upper insulator layer to a green foil, i.e., an unsintered ceramic foil, using thin layer or thick layer technology. The green foil including the printed-on heating device is subsequently laminated together with other green foils onto which electrodes may be printed, for example, and is sintered.
The insulator is often porous. The porosity of the insulator is achieved by adding a pore former prior to sintering. During sintering, the pore former is burned away, so that a porous structure is obtained. The porosity is adjusted via the amount of pore former, for example, carbon glass, which is added.
German Published Patent Application No. 198 53 601 furthermore describes a broadband lambda probe for determining the oxygen concentration in exhaust gases of internal combustion engines. The sensor element of the probe includes a heating device, which is electrically insulated from a solid electrolyte layer by an insulator. The insulator contains a mixture of aluminum oxide, barium oxide, and/or strontium oxide to prevent leak currents.
The disadvantage of such a sensor element is that the sintering characteristics, in particular sintering shrinkage, of the solid electrolyte layer and the adjacent barium and/or strontium-containing insulator are distinctly different. The thermal expansion coefficients of the insulator and the solid electrolyte layer are also different. The poor bonding of the insulator to the solid electrolyte layer results in cracks, for example, in the event of rapid temperature changes, which impair the functionality of the sensor element.